James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991.[22] Java was originally designed for interactive television, but it was too advanced for the digital cable television industry at the time.[23] The language was initially called Oak after an oak tree that stood outside Gosling's office. Later the project went by the name Green and was finally renamed Java, from Java coffee.[24] Gosling designed Java with a C/C++-style syntax that system and application programmers would find familiar.[25]

Sun Microsystems released the first public implementation as Java 1.0 in 1995.[26] It promised "Write Once, Run Anywhere" (WORA), providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java applets within web pages, and Java quickly became popular, while mostly outside of browsers, that wasn't the original plan. In January 2016, Oracle announced that Java runtime environments based on JDK 9 will discontinue the browser plugin.[27] The Java 1.0 compiler was re-written in Java by Arthur van Hoff to comply strictly with the Java 1.0 language specification.[28] With the advent of Java 2 (released initially as J2SE 1.2 in December 1998 – 1999), new versions had multiple configurations built for different types of platforms. J2EE included technologies and APIs for enterprise applications typically run in server environments, while J2ME featured APIs optimized for mobile applications. The desktop version was renamed J2SE. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE, Java ME, and Java SE, respectively.

On November 13, 2006, Sun released much of its Java virtual machine (JVM) as free and open-source software, (FOSS), under the terms of the GNU General Public License (GPL). On May 8, 2007, Sun finished the process, making all of its JVM's core code available under free software/open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.[33]

Sun's vice-president Rich Green said that Sun's ideal role with regard to Java was as an "evangelist".[34] Following Oracle Corporation's acquisition of Sun Microsystems in 2009–10, Oracle has described itself as the "steward of Java technology with a relentless commitment to fostering a community of participation and transparency".[35] This did not prevent Oracle from filing a lawsuit against Google shortly after that for using Java inside the Android SDK (see Google section below). Java software runs on everything from laptops to data centers, game consoles to scientific supercomputers.[36] On April 2, 2010, James Gosling resigned from Oracle.[37]

Principles

There were five primary goals in the creation of the Java language:[16]

Practices

Java platform

One design goal of Java is portability, which means that programs written for the Java platform must run similarly on any combination of hardware and operating system with adequate runtime support. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to architecture-specific machine code. Java bytecode instructions are analogous to machine code, but they are intended to be executed by a virtual machine (VM) written specifically for the host hardware. End users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a web browser for Java applets.

Standard libraries provide a generic way to access host-specific features such as graphics, threading, and networking.

The use of universal bytecode makes porting simple. However, the overhead of interpreting bytecode into machine instructions makes interpreted programs almost always run more slowly than native executables. However, just-in-time (JIT) compilers that compile bytecodes to machine code during runtime were introduced from an early stage. Java itself is platform-independent, and is adapted to the particular platform it is to run on by a Java virtual machine for it, which translates the Java bytecode into the platform's machine language.[39]

Implementations

Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Microsystems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Microsoft Windows (still works for XP, while only later versions currently officially supported), Mac OS X, Linux and Solaris. Because Java lacks any formal standardization recognized by Ecma International, ISO/IEC, ANSI, or other third-party standards organization, the Oracle implementation is the de facto standard.

The Oracle implementation is packaged into two different distributions: The Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs and is intended for end users, and the Java Development Kit (JDK), which is intended for software developers and includes development tools such as the Java compiler, Javadoc, Jar, and a debugger.

OpenJDK is another notable Java SE implementation that is licensed under the GNU GPL. The implementation started when Sun began releasing the Java source code under the GPL. As of Java SE 7, OpenJDK is the official Java reference implementation.

The goal of Java is to make all implementations of Java compatible. Historically, Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun.[40] As a result, Microsoft no longer ships Java with Windows.

Platform-independent Java is essential to Java EE, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications.

Performance

Programs written in Java have a reputation for being slower and requiring more memory than those written in C++.[41][42] However, Java programs' execution speed improved significantly with the introduction of just-in-time compilation in 1997/1998 for Java 1.1,[43] the addition of language features supporting better code analysis (such as inner classes, the StringBuilder class, optional assertions, etc.), and optimizations in the Java virtual machine, such as HotSpot becoming the default for Sun's JVM in 2000. With Java 1.5, the performance was improved with the addition of the java.util.concurrent package, including Lock free implementations of the ConcurrentMaps and other multi-core collections, and it was improved further Java 1.6.

Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software Java virtual machine, and ARM based processors can have hardware support for executing Java bytecode through their Jazelle option (while its support is mostly dropped in current implementations of ARM).

Automatic memory management

Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.[44][45]

One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e., those where the memory is still referenced but never used.

Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.

Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.

As in C++ and some other object-oriented languages, variables of Java's primitive data types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as is commonly true for non-primitive data types (but see escape analysis). This was a conscious decision by Java's designers for performance reasons.

Syntax

The syntax of Java is largely influenced by C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language.[16] All code is written inside classes, and every data item is an object, with the exception of the primitive data types, i.e. integers, floating-point numbers, boolean values, and characters, which are not objects for performance reasons. Java reuses some popular aspects of C++ (such as printf() method).

Java uses comments similar to those of C++. There are three different styles of comments: a single line style marked with two slashes (//), a multiple line style opened with /* and closed with */, and the Javadoc commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to create documentation for the program.

Example:

// This is an example of a single line comment using two slashes/* This is an example of a multiple line comment using the slash and asterisk. This type of comment can be used to hold a lot of information or deactivate code, but it is very important to remember to close the comment. */packagefibsandlies;importjava.util.HashMap;/** * This is an example of a Javadoc comment; Javadoc can compile documentation * from this text. Javadoc comments must immediately precede the class, method, or field being documented. */publicclassFibCalculatorextendsFibonacciimplementsCalculator{privatestaticMap<Integer,Integer>memoized=newHashMap<Integer,Integer>();/* * The main method written as follows is used by the JVM as a starting point for the program. */publicstaticvoidmain(String[]args){memoized.put(1,1);memoized.put(2,1);System.out.println(fibonacci(12));//Get the 12th Fibonacci number and print to console}/** * An example of a method written in Java, wrapped in a class. * Given a non-negative number FIBINDEX, returns * the Nth Fibonacci number, where N equals FIBINDEX. * @param fibIndex The index of the Fibonacci number * @return The Fibonacci number */publicstaticintfibonacci(intfibIndex){if(memoized.containsKey(fibIndex)){returnmemoized.get(fibIndex);}else{intanswer=fibonacci(fibIndex-1)+fibonacci(fibIndex-2);memoized.put(fibIndex,answer);returnanswer;}}}

Examples

"Hello, world!" program

classHelloWorldApp{publicstaticvoidmain(String[]args){System.out.println("Hello World!");// Prints the string to the console.}}

Source files must be named after the public class they contain, appending the suffix .java, for example, HelloWorldApp.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorldApp.class. Only then can it be executed, or "launched". The Java source file may only contain one public class, but it can contain multiple classes with other than public access and any number of public inner classes. When the source file contains multiple classes, make one class "public" and name the source file with that public class name.

A class that is not declared public may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name were the concatenation of the name of their enclosing class, a $, and an integer.

The keywordpublic denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is located. This is called an access level modifier. Other access level modifiers include the keywords private and protected.

The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any class members that are not also static. Methods that are not designated static are instance methods, and require a specific instance of a class to operate.

The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly.

The method name "main" is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise JavaBeans do not use or need a main() method. A Java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from.

The main method must accept an array of String objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array of String objects), but it allows an alternative syntax for creating and passing the array.

The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.

Printing is part of a Java standard library: The System class defines a public static field called out. The out object is an instance of the PrintStream class and provides many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.

The string "Hello World!" is automatically converted to a String object by the compiler.

// OddEven.javaimportjavax.swing.JOptionPane;publicclassOddEven{privateintuserInput;// a whole number("int" means integer)/** * This is the constructor method. It gets called when an object of the OddEven type * is being created. */publicOddEven(){/* * In most Java programs constructors can initialize objects with default values, or create * other objects that this object might use to perform its functions. In some Java programs, the * constructor may simply be an empty function if nothing needs to be initialized prior to the * functioning of the object. In this program's case, an empty constructor would suffice. * A constructor must exist; however, if the user doesn't put one in then the compiler * will create an empty one. */}/** * This is the main method. It gets called when this class is run through a Java interpreter. * @param args command line arguments (unused) */publicstaticvoidmain(finalString[]args){/* * This line of code creates a new instance of this class called "number" (also known as an * Object) and initializes it by calling the constructor. The next line of code calls * the "showDialog()" method, which brings up a prompt to ask you for a number. */OddEvennumber=newOddEven();number.showDialog();}publicvoidshowDialog(){/* * "try" makes sure nothing goes wrong. If something does, * the interpreter skips to "catch" to see what it should do. */try{/* * The code below brings up a JOptionPane, which is a dialog box * The String returned by the "showInputDialog()" method is converted into * an integer, making the program treat it as a number instead of a word. * After that, this method calls a second method, calculate() that will * display either "Even" or "Odd." */userInput=Integer.parseInt(JOptionPane.showInputDialog("Please enter a number."));calculate();}catch(finalNumberFormatExceptione){/* * Getting in the catch block means that there was a problem with the format of * the number. Probably some letters were typed in instead of a number. */System.err.println("ERROR: Invalid input. Please type in a numerical value.");}}/** * When this gets called, it sends a message to the interpreter. * The interpreter usually shows it on the command prompt (For Windows users) * or the terminal (For *nix users).(Assuming it's open) */privatevoidcalculate(){if((userInput%2)==0){JOptionPane.showMessageDialog(null,"Even");}else{JOptionPane.showMessageDialog(null,"Odd");}}}

The OddEven class declares a single privatefield of type int named userInput. Every instance of the OddEven class has its own copy of the userInput field. The private declaration means that no other class can access (read or write) the userInput field.

OddEven() is a publicconstructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class.

The calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression userInput % 2 == 0 using the if keyword to see if the remainder of dividing the userInput field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (The calculate method can be equivalently accessed as this.calculate and the userInput field can be equivalently accessed as this.userInput, which both explicitly use the undeclared this parameter.)

OddEven number = new OddEven(); declares a local object reference variable in the main method named number. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEven class, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.

The statement number.showDialog(); calls the calculate method. The instance of OddEven object referenced by the numberlocal variable is used to invoke the method and passed as the undeclared this parameter to the calculate method.

userInput = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive data typeint by using a utility function in the primitive wrapper classInteger.

The import statements direct the Java compiler to include the javax.swing.JApplet and java.awt.Graphics classes in the compilation. The import statement allows these classes to be referenced in the source code using the simple class name (i.e. JApplet) instead of the fully qualified class name (FQCN, i.e. javax.swing.JApplet).

The Hello class extends (subclasses) the JApplet (Java Applet) class; the JApplet class provides the framework for the host application to display and control the lifecycle of the applet. The JApplet class is a JComponent (Java Graphical Component) which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.

The Hello class overrides the paintComponent(Graphics) method (additionally indicated with the annotation, supported as of JDK 1.5, Override) inherited from the Containersuperclass to provide the code to display the applet. The paintComponent() method is passed a Graphics object that contains the graphic context used to display the applet. The paintComponent() method calls the graphic context drawString(String, int, int) method to display the "Hello, world!" string at a pixel offset of (65, 95) from the upper-left corner in the applet's display.

An applet is placed in an HTML document using the <applet>HTML element. The applet tag has three attributes set: code="Hello" specifies the name of the JApplet class and width="200" height="200" sets the pixel width and height of the applet. Applets may also be embedded in HTML using either the object or embed element,[51] although support for these elements by web browsers is inconsistent.[52] However, the applet tag is deprecated, so the object tag is preferred where supported.

The host application, typically a Web browser, instantiates the Hello applet and creates an AppletContext for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The paintComponent() method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.

Servlet

Java Servlet technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems. Servlets are server-side Java EE components that generate responses (typically HTML pages) to requests (typically HTTP requests) from clients. A servlet can almost be thought of as an applet that runs on the server side—without a face.

The import statements direct the Java compiler to include all the public classes and interfaces from the java.io and javax.servlet packages in the compilation. Packages make Java well suited for large scale applications.

The Hello class extends the GenericServlet class; the GenericServlet class provides the interface for the server to forward requests to the servlet and control the servlet's lifecycle.

The setContentType(String) method in the response object is called to set the MIME content type of the returned data to "text/html". The getWriter() method in the response returns a PrintWriter object that is used to write the data that is sent to the client. The println(String) method is called to write the "Hello, world!" string to the response and then the close() method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.

JavaServer Pages

JavaServer Pages (JSP) are server-side Java EE components that generate responses, typically HTML pages, to HTTP requests from clients. JSPs embed Java code in an HTML page by using the special delimiters<% and %>. A JSP is compiled to a Java servlet, a Java application in its own right, the first time it is accessed. After that, the generated servlet creates the response.

Swing application

Swing is a graphical user interface library for the Java SE platform. It is possible to specify a different look and feel through the pluggable look and feel system of Swing. Clones of Windows, GTK+ and Motif are supplied by Sun. Apple also provides an Aqua look and feel for Mac OS X. Where prior implementations of these looks and feels may have been considered lacking, Swing in Java SE 6 addresses this problem by using more native GUI widget drawing routines of the underlying platforms.

This example Swing application creates a single window with "Hello, world!" inside:

The Hello()constructor initializes the frame by first calling the superclass constructor, passing the parameter "hello", which is used as the window's title. It then calls the setDefaultCloseOperation(int) method inherited from JFrame to set the default operation when the close control on the title bar is selected to WindowConstants.EXIT_ON_CLOSE – this causes the JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the Java virtual machine to exit and the program to terminate. Next, a JLabel is created for the string "Hello, world!" and the add(Component) method inherited from the Container superclass is called to add the label to the frame. The pack() method inherited from the Window superclass is called to size the window and lay out its contents.

The main() method is called by the Java virtual machine when the program starts. It instantiates a new Hello frame and causes it to be displayed by calling the setVisible(boolean) method inherited from the Component superclass with the boolean parameter true. Once the frame is displayed, exiting the main method does not cause the program to terminate because the AWT event dispatching thread remains active until all of the Swing top-level windows have been disposed.

Generics

In 2004, generics were added to the Java language, as part of J2SE 5.0. Prior to the introduction of generics, each variable declaration had to be of a specific type. For container classes, for example, this is a problem because there is no easy way to create a container that accepts only specific types of objects. Either the container operates on all subtypes of a class or interface, usually Object, or a different container class has to be created for each contained class. Generics allow compile-time type checking without having to create many container classes, each containing almost identical code. In addition to enabling more efficient code, certain runtime exceptions are converted to compile-time errors, a characteristic known as type safety.

Criticism

Criticisms directed at Java include the implementation of generics,[53] speed,[54] the handling of unsigned numbers,[55] the implementation of floating-point arithmetic,[56] and a history of security vulnerabilities in the primary Java VM implementation HotSpot.[57]

Use outside of the Java platform

The Java programming language requires the presence of a software platform in order for compiled programs to be executed. Oracle supplies the Java platform for use with Java. The Android SDK, is an alternative software platform, used primarily for developing Android applications.

Android

The Android operating system makes extensive use of Java-related technology.

Android does not provide the full Java SE standard library, although the Android SDK does include an independent implementation of a large subset of it. It supports Java 6 and some Java 7 features, offering an implementation compatible with the standard library (Apache Harmony).

Controversy

The use of Java-related technology in Android led to a legal dispute between Oracle and Google. On May 7, 2012, a San Francisco jury found that if APIs could be copyrighted, then Google had infringed Oracle's copyrights by the use of Java in Android devices.[58] District Judge William Haskell Alsup ruled on May 31, 2012, that APIs cannot be copyrighted,[59] but this was reversed by the United States Court of Appeals for the Federal Circuit in May 2014.[60] On May 26, 2016, the district court decided in favor of Google, ruling the copyright infringement of the Java API in Android constitutes fair use.[61]

The Java Class Library is the standard library, developed to support application development in Java. It is controlled by Sun Microsystems in cooperation with others through the Java Community Process program. Companies or individuals participating in this process can influence the design and development of the APIs. This process has been a subject of controversy.[when?] The class library contains features such as:

Documentation

Javadoc is a comprehensive documentation system, created by Sun Microsystems, used by many Java developers[by whom?]. It provides developers with an organized system for documenting their code. Javadoc comments have an extra asterisk at the beginning, i.e. the delimiters are /** and */, whereas the normal multi-line comments in Java are set off with the delimiters /* and */.[65]

^Niklaus Wirth stated on a number of public occasions, e.g. in a lecture at the Polytechnic Museum, Moscow in September, 2005 (several independent first-hand accounts in Russian exist, e.g. one with an audio recording: Filippova, Elena (September 22, 2005). "Niklaus Wirth's lecture at the Polytechnic Museum in Moscow".), that the Sun Java design team licensed the Oberon compiler sources a number of years prior to the release of Java and examined it: a (relative) compactness, type safety, garbage collection, no multiple inheritance for classes – all these key overall design features are shared by Java and Oberon.

^Patrick Naughton cites Objective-C as a strong influence on the design of the Java programming language, stating that notable direct derivatives include Java interfaces (derived from Objective-C's protocol) and primitive wrapper classes. [3]

^TechMetrix Research (1999). "History of Java"(PDF). Java Application Servers Report. The project went ahead under the name "green" and the language was based on an old model of UCSD Pascal, which makes it possible to generate interpretive code

^In the summer of 1996, Sun was designing the precursor to what is now the event model of the AWT and the JavaBeans TM component architecture. Borland contributed greatly to this process. We looked very carefully at Delphi Object Pascal and built a working prototype of bound method references in order to understand their interaction with the Java programming language and its APIs.White Paper About Microsoft's "Delegates"

^McMillan, Robert (2013-08-01). "Is Java Losing Its Mojo?". wired.com. Java is on the wane, at least according to one outfit that keeps on eye on the ever-changing world of computer programming languages. For more than a decade, it has dominated the TIOBE Programming Community Index, and is back on top – a snapshot of software developer enthusiasm that looks at things like internet search results to measure how much buzz different languages have. But lately, Java has been slipping.

^"Oracle and Java". oracle.com. Oracle Corporation. Retrieved 2010-08-23. Oracle has been a leading and substantive supporter of Java since its emergence in 1995 and takes on the new role as steward of Java technology with a relentless commitment to fostering a community of participation and transparency.